Field of the Invention
The present invention relates to the general technical field of engines or machines with permanent magnets which are intended to generate an electric current.
The present invention particularly relates to a magnet-bearing moving part for a permanent magnet synchronous machine, such as a machine of the motor or generator type or any other rotating electrical machine or non-rotating permanent magnet machine.
A preferred application of the invention relates more specifically to a traction motor rotor, for example a rail traction motor. Another application concerns, for example, various types of road vehicles.
Description of Related Art
The permanent magnet synchronous machines comprise a moving part with a series of permanent magnets of alternating polarity and a fixed part called a stator comprising a set of induction coils.
Depending on the type of synchronous machines, the magnet-bearing moving part can move linearly relative to the stator or rotate relative thereto. In the latter case, it is called a rotor.
To generate an induction phenomenon, which sets in motion the moving part in the case of a motor or creates a current in the case of a generator, these moving parts comprise successive rows of permanent magnets positioned vis-à-vis the stator windings. These rows, parallel to each other, are conventionally oriented perpendicularly to the movement (that is to say, in the axial direction in the case of circular movement). For example, the magnets have the same polarity within the same row, but have alternating polarity from one row to the next. It is thus also possible to produce alternating polarities with magnets of the same polarity arranged in two or three successive rows.
Conventionally, the moving parts comprise a metal frame on which the magnets are secured by adhesion. To ensure satisfactory maintenance, the adhesive used must, however, have features that are compatible with the major stresses that the magnets are subjected to, regardless of the operating temperature.
Indeed, during the use of the synchronous machine, these magnets are subjected to multiple stresses acting in various directions. This refers to, for example, in the case of a rotating rotor, the following: axial stresses resulting from the magnetic attractions and repulsions between the magnets, radial stresses associated with the closure of the magnetic field of the stator, centrifugal stresses due to more or less rapid rotation of the rotor and tangential shear stresses due to the torque.
Currently, the tendency is to want to create synchronous machines that are increasingly powerful, while accommodating them in an increasingly smaller space. This commitment to improving the performance of synchronous machine has simultaneously led to a significant increase in the internal operating temperatures of these machines.
The adhesives used for securing the magnets on the moving part of these machines must thus be able to withstand such temperatures, while ensuring good performance of the magnets despite significant constraints. Such high temperature performance levels are difficult to obtain for an adhesive.
In addition, this increase in the internal operating temperature of the synchronous machines causes a structural expansion that is not of the same intensity as the respective composition of each part. A differential expansion is thus observed between the frame of the moving part, which is usually made of steel, and the magnets which are, for example, made of neodymium iron boron.
The adhesives used in order to ensure that the magnets are properly secured are rigid adhesives that generally do not have sufficient elasticity to be compatible with this differential expansion.
In the event of deformation of the moving part caused by such differences in expansion or by external constraints or by thermal runaway, for example, following a converter dysfunction, a rupture in the adhesive film is sometimes observed between the frame and the magnets, which can cause all or part of the magnets on the stator to slide. Such a rupture results in the sudden loss of performance of the synchronous machine, or even in a complete blockage thereof.
Another drawback of these synchronous machines is the abrupt alternation of the polarities of permanent magnets arranged in parallel rows during the movement of the moving part in front of the stator. This sudden change of North/South polarity of the magnets causes an oscillation of the torque with a very steep slope that generates vibrations and jolts in the gears, transmissions and all mechanical components. These vibrations, in addition to some passenger discomfort related to the noise generated, lead to premature wear or damage to the mechanical components.
In the case of a preferred application of these synchronous machines with motor-wheels, these sudden torque oscillations cause premature tyre wear.
In order to reduce the torque ripple due to the abrupt transition from one polarity to another, it has been proposed in the prior art that the magnets should not be positioned in rows perpendicular to the movement, but according to a helical arrangement. With such a helical arrangement, the transition between polarities of the successive rows of magnets is more gradual during the movement of the moving part. The torque oscillations are therefore absorbed.
In addition to or instead of this helical arrangement of the magnets of the movable part, it was also considered in the prior art that a helical arrangement should be implemented for the stator windings.
However, this particular helical arrangement of the magnets of the moving part and/or the stator coils is quite difficult to achieve. The installation of these elements, already difficult due to magnetic interactions between the magnets, becomes particularly complex and sensitive. The manufacture of these synchronous machines is complicated, lengthy and expensive.
In addition, because of this helical arrangement, the performance of the synchronous machine is significantly reduced.